The present invention is directed to a light waveguide comprising three layers of plastic material applied to an optical fiber and to the method of manufacturing the waveguide.
U.S. Pat. No. 4,334,733, whose disclosure is incorporated by reference and which claims priority from the same Japanese application as German Patent No. 30 02 362, discloses an optical fiber which has three layers applied to the fiber. An innermost layer or primary coating is composed of a resin coat, which is applied to the bare fiber immediately after drawing the optical fiber by drawing the bare fiber through a cell or container which has the liquid resin mixture. The thickness of this primary coat lies roughly in the range of 10 and 30 .mu.m. A subsequent buffer layer is applied to this primary layer and can be composed of a silicon resin, rubber-like material or the like, and has a wall thickness of between 50 and 200 .mu.m. On the outside of the buffer layer is a third coating or layer, which is a secondary coating that is composed of a relatively hard material and is applied in a melt or flow extrusion method. The planned or desired outside diameter of the light waveguides coated in this way lie in the order of magnitude of approximately 900 .mu.m. This involves a matter of a permanently or firmly enveloped fiber for which the outside diameter is typical.
Devices, that must be very well centered, are needed for the application of the innermost or primary coating. On the other hand, the optical fiber here is still completely unprotected so that guide devices or the like are difficult to use. Moreover, this method is highly dependent on the quality of the centering and damage to or deterioration of the optical fibers can occur, given inadequate centering, particularly when a low wall thickness in the coating is provided. In addition, the viscosity of the resin compound used for the first coating plays a particular part, and this requires that the cell must be appropriately heated. In the known coating with resin compounds in a cell, the applied pressure is of a great influence in order to shorten the draw-in cone in the immediate proximity of the fiber. When this is not observed, gas can be encapsulated by the coating. Another disadvantage of the known arrangement is that, given a modification of the drawing rate, all of the parameters cited above in the primary coating must also be changed, and this potentially leads to considerable adjustment problems.